In a power system, an overcurrent relay is used to transfer a failure current from a failed power line, for example, into a signal instructing a circuit breaker to disconnect the failed power line from the power system so as to protect the whole power system from being damaged by the failure current. FIG. 1 is a schematic block diagram showing a general protection circuit 10 including a relay 15. In the protection circuit 10, as commonly known, a current transformer (CT) 13 is used to detect a failure current from an AC (Alternating Current) bus 21. When a failure occurs in an apparatus connected with the AC bus 21, a great current may flow through the AC bus 21 and results in damage to a power system (not shown) connected with the AC bus 21. The relay 15 transfers the failure current detected by the CT 13 into a signal and passes the signal to a circuit breaker 17. The circuit breaker 17 then operates to isolate the AC bus from the power system.
As known, the CT 13 has an iron core. Accordingly, a magnetic hysteresis phenomenon exists. When detecting a great overcurrent, a distortion may occur in the sensed current waveform due to a non-liner CT-exciting current, which leads to saturation of the magnetic flux in the CT 13. Such a distortion may cause a time delay to the cut-off of the circuit breaker 17.
FIG. 2 shows protection curves of different conventional relays. In this plot, the horizontal axis is the current level, and the vertical axis is the cut off trip time. As can be seen, the protection curves are parabolic curves. Different relay characteristics may result in parabolic curves with different curvatures. When a power system utilises various relays manufactured by different manufacturers, to avoid interference between the protection curves, a considerable time interval must be kept between two adjacent protection curves with respect to the diversity of the relays. When a distortion of the CT sensing occurs, an improper circuit breaker may be driven to operate due to the time delay resulted from the CT sensing distortion.
FIG. 3 shows ideal three-phase CT current waveforms during a power line failure. As can been seen, for each phase, an envelope of the CT current should have a smooth curve. FIG. 4 shows a distorted CT current waveform in one phase during the same power line failure. As shown, the envelope is significantly deformed.
As mentioned above, the magnetic saturation of the CT 13 leads to a time delay, and therefore the relay 15 cannot respond to the failure current immediately. Accordingly, power supplying quality and system safety are both affected. It will be high appreciated if a solution can be provided to solve the above problem.